The present invention relates to a sheet feeding apparatus that feeds and transports a transfer sheet to a transfer processing unit in an image forming apparatus such as an electrophotographic copying machine, and more particularly to a sheet feeding apparatus that is positioned immediately before a transfer processing unit to feed out a transfer sheet after synchronizing the transfer sheet with the transfer processing unit by regulating the position and direction of the transfer sheet.
For the purpose of timely transfer of toner images formed on a photoreceptor, a transfer sheet in an electrophotographic copying machine is stopped temporarily at registration rollers positioned immediately before the transfer processing unit in the conveyer path to the photoreceptor where a position and a conveyance timing of said transfer sheet are adjusted. Then, the registration rollers are rotated in synchronization with the rotation of the photoreceptor to move the transfer sheet that is nipped between the registration rollers to a transfer area after optical scanning is started and toner images are formed on the photoreceptor.
At the registration rollers, the leading edge of the transfer sheet transported by buckling rollers 10 positioned at the upstream side is caused to hit the registration rollers to stop. Even after the transfer sheet is stopped, the buckling rollers keep rotating for a predetermined time so that a bulge of the transfer sheet may be formed. Utilizing the elasticity of the bulge, the leading edge of the transfer sheet is inserted into the nip portion of the registration rollers which are to be started, and then the buckling rollers and the registration rollers are rotated synchronously relative to each other to convey move the transfer sheet. Therefore, a substantial part of the bulge formed by the transfer sheet excluding the portion inserted when the registration rollers are rotated remains as it is. Thus the registration rollers and the buckling rollers convey the transfer sheet while keeping the bulge.
However, the bulge of the transfer sheet is not necessarily formed uniformly. Therefore, there has been a fear that the transfer sheet will receive unnecessary stress when it is transported while the bulge is kept, and that a plosive noise is produced when the transfer sheet is pulled. Further, when jamming takes place in a conventional sheet feeding apparatus, buckling rollers have needed to be released manually from their pressure contact for clearing the jammed sheet and therefore the sheet feeding apparatus has needed to be opened for each occurrence of a jam.
An object of the invention is to provide a sheet feeding apparatus wherein pressure contact of buckling rollers is released on a timely basis, thereby a bulge of a transfer sheet is eliminated, the transfer sheet is free from unnecessary stress, the noise level is low and jam clearance is easy.